1. Field of the Invention
The present invention relates to a process for producing methyl lactate using vinyl acetate or vinyl propionate as a starting material.
2. Description of the Prior Art
Methyl lactate is used in significant quantities as a raw material for the production of lactic acid. Currently, lactic acid is commercially produced by the following route. ##STR1## However, that method has the following disadvantages: (1) The starting materials, i.e., acetaldehyde and hydrogen cyanide, are likely to be less readily available and only at increased costs in the near future. (2) Hydrogen cyanide and its derivatives, which are highly toxic, must be treated by a costly process in order to dispose of the same to meet pollution control regulations. (3) Reaction (i) must be conducted at a sufficiently low temperature and the yield of the acetaldehyde cyanohydrin is not necessarily high. (4) Reaction (ii) is very exothermic and requires an effective cooling system which adversely affects the stability of the operation. The use of sulfuric acid at high temperatures necessitates the use of a reactor fabricated from expensive construction materials. (5) In reaction (iii), crystalline ammonium bisulfate which is difficult to work with is by-produced along with methyl lactate in equimolar proportions. Moreover, methyl methoxylactate, carbon monoxide, tarry substances, and the like are also by-produced in appreciable quantities. Though the formation of these by-products can be held within allowable limits by introducing large amounts of methanol and water into the reaction system, such a practice will result in increased energy consumption and, hence, an increase in the price of the final product. (6) In terms of overall yield, the yield of lactic acid is not necessarily high and large amounts of steam are consumed. Moreover, the method is not suitable for continuous production.
An alternative method has been proposed for the production of lactic acid which comprises hydroformylating a vinyl carboxylate to give an .alpha.-acyloxypropionaldehyde, oxidizing the same with oxygen to give an .alpha.-acyloxypropionic acid and finally hydrolyzing the same to lactic acid (cf. U.S. Pat. No. 4,072,709). However, this method has the following disadvantages. (1) In the initial hydroformylation reaction, the separation of .alpha.-acyloxypropionaldehyde from the reaction mixture is effected by distillation. However, (a) the .alpha.-acyloxypropionaldehyde undergoes decarboxylation, polycondensation, oxidation and/or other undesirable side reactions under the conditions of distillation, and (b) the repetition of hydroformylation followed by separation by distillation results in a loss in the catalytic activity of the rhodium complex recycled and reused and at the same time high-boiling by-products accumulate. (2) Since the .alpha.-acyloxypropionic acid obtained by oxidizing .alpha.-acyloxypropionaldehyde with oxygen contains impurities, the direct hydrolysis of the reaction product results in a lactic acid product of decreased purity. It is possible to purify the .alpha.-acyloxypropionic acid by distillation beforehand, however, because of its thermal instability, decreased yields of .alpha.-acyloxypropionic acid product are obtained. (3) The reaction mixture obtained by hydrolyzing .alpha.-acyloxypropionic acid contains, not only lactic acid, but also water, the organic carboxylic acid derived from the vinyl carboxylate used, and unreacted .alpha.-acyloxypropionic acid. It is difficult to recover lactic acid in high purity from such a reaction mixture. A need, therefore, continues to exist for an improved method of preparing methyl lactate in high yields.